Rheology Modifiers.
Rheology modifiers are useful for adjusting liquid shear viscosity as a function of shear rate. Latex paints, for example, must have suitable viscosity at high shear rates to give correct coverage and brush drag. They must also, however, have suitable viscosity at low shear rates so as not to sag on vertical surfaces while permitting leveling of coating defects.
The adjustment of viscosity in water-borne latex paints is often accomplished by the addition of such materials as alkali swellable thickeners, lower molecular weight associative thickeners, water soluble polymers, and the like.
See also Soules, et al. "Dynamic Uniaxial Extensional Viscosity", Chapter 20, pp 322-332 in Polymers as Rheology Modifiers, D. N. Schulz and J. E. Glass eds., in American Chemical Society Symposium Series #462 (1991), which describes the use of additives to affect various rheological properties of water-bome latex paint coatings, and Soules, et al. J. Rheology 32(2): 181-198 ( 1988) which describes the use of fillers, including poly(oxyethylene), to affect the extensional viscosity of formulations for spray applications.
Two types of water-based rheology modifiers are commonly used to increase shear viscosity, and are often referred to as "thickeners". The first type includes alkali-activated copolymers of acrylic or methacrylic acid monomer and alkyl (meth)acrylates or butyl acrylate monomer. Thickening is achieved when initially acidic solutions are neutralized to pH greater than about 6.5 and the acid groups become soluble, causing swelling and interaction of the polymer chains.
The second type of thickeners, referred to as associative thickeners, are becoming increasingly popular in the paint industry for providing desired "brush drag" and leveling properties. These thickeners are similar to the alkali-activated copolymers, but include a grafted nonionic surfactant to provide physical intermolecular crosslinks.
Commercial HMW PEO resins are said to be extremely effective thickening agents in both fresh and salt water. The aqueous solutions are described as pseudoplastic (i.e., "shear thinning"). (See, e.g., "POLYOX Water Soluble Resins", Union Carbide Product literature, 12 pgs., 1991, 1992)
Glass, "Dynamics of Roll Spatter and Tracking", J. Coatings Tech. 50(641):56-71 (1978) describes the addition of water soluble polymers, including high molecular weight poly(ethylene oxide) polymers, to latex paint in order to study their effect on such properties as spatter and tracking patterus.
Others have described the use of high or low molecular weight epoxide polymers in water-based systems. See, for example, U.S. Pat. No. 3,944,100 (Brockway, et at.) which describes a "containment" coating composition for glass containers, the composition including the optional use of 100-800 ppm PEO, which can be added for shear enhancement.
U.S. Pat. No. 4,507,413 (Thoma et at.) describes a spreadable polyurethane reactive coating composition for coating textile sheet products and the like. The patent describes the optional use of substances for thickening the spread coating viscosity of an aqueous dispersion of blocked isocyanate prepolymer. Included in the group of thickening substances are water soluble polymers, including long chain polyethylene oxides.
Schuppiser et al., in U.S. Pat. No. 4,542,182 describe the use of ethylene glycol polymer having low molecular weights (MW of between 1,000 and 50,000) for preparing lattices of particles of vinyl acetate olefin copolymers. The lattices, in turn, are applied as binders in the manufacture of paints, and other materials.
U.S. Pat. No. 5,108,782 to Reed describes a silicone release coating emulsion that includes the use of a water soluble or water dispersible polymeric thickening agent such as high molecular weight PEO. The silicone emulsion is said to exhibit improved holdout of the silicone from the substrate.
U.S. Pat. No. 5,342,883 (Jenkins, et al.) discloses the use of water soluble polymers formed from alpha, beta-monoethylenically unsaturated monomers. The polymers are either soluble in or swelled by an aqueous alkaline medium. When added to latex paints and neutralized, both the viscosity of the paint and brush drag are said to be increased.
High molecular weight polymers such as poly(ethylene oxides) have not previously been used or described as useful for improving the rheological properties of materials such as water-based pressure sensitive adhesives.
Adhesive Coating Techniques.
Premetered liquid coaters include those in which the flow rate of the coating material is controlled before its application to a substrate. Such coaters, which include slot coaters, slide coaters, and curtain coaters, can be used to deliver thin sheets of materials such as adhesives to a substrate. In each situation, a fluid is fed to a coating die, and in turn is applied to a substrate in the form of a stable sheet (also known as a "bead" in the context of slot coating).
With other (e.g., postmetered) coating techniques, such as dip coaters and roll coaters, the amount of fluid applied to the substrate depends on the fluid properties themselves, such as fluid viscosity and surface tension, as well as on the operating conditions such as substrate speed and coating gap.
Typically a premetered coater provides a coating die for the delivery of material, and provides for a physical gap between the die and substrate. With slot and slide coaters, the gap is generally more than 100 microns (about 4 mils) in distance, while with curtain coaters the gap can be on the order of centimeters.
The combination of flow rate per unit width and substrate speed together determine the thickness of the delivered sheet, and in turn of the coating itself. In order to obtain thinner coatings, it is often more desirable to decrease the flow rate rather than increase the substrate speed, since the latter is generally more prone to problems with flow stability and drying limitations. The ability to decrease flow rate, however, is generally limited by the rheological properties of the coating composition itself. Lowering a flow rate too much will result in the application of non-uniform or unstable sheets.
Since coating thickness is also known to correlate with the size of the coating gap between the die and substrate, there is a strong tendency in the extrusion coating industry to use very tight gaps to produce very thin coatings. However gaps under about 100 microns frequently result in operating difficulties, since particulate matter that accumulates in the coating gap can cause streaks. Tight gaps also make it easy for large particles or taped splices in the substrate to tear the substrate (e.g., web). As a result, most conventional slot coating processes continue to rely on gaps greater than about 50 microns.
Pressure-sensitive adhesive latexes are typically thickened for best handling on coating equipment. The shear thickeners typically used have included cellulosic thickeners, poly(vinyl alcohol) and aqueous solutions of alkali soluble acrylic emulsions. (See, e.g., D. Satas, Chapter 13, Handbook of Pressure-Sensitive Adhesive Technology, VanNostrand Reinhold (1982).
It is clear that the industry has a need for methods for improving the coatability of pressure-sensitive adhesive formulations, and in particular, for decreasing the flow rate, or increasing the coating gap in the course of premetered coating.